GB2090291A - Sputter ion plating of refractory metal/metal compounds - Google Patents

Abstract

To form smooth dense pore-free coatings without developing high substrate temperatures the source material consists of said refractory metal such as tungsten, tantalum or metal compound and one or more further metals e.g. another refractory metal and the substrate bias potential is reduced compared to the bias potential required for plating said single refractory metal or metal compound alone. The source material may be a bundle of wires of the refractory metal or metal compound and wires of the further metal and preferably consists of the refractory metal or metal compound and the further metal in proportion to form a solid solution on the substrate. A gas capable of reacting chemically with the refractory metal or the further metal may be present. For example, a hard refractory nitride may be formed using a reactive gas mixture of nitrogen in argon and tantalum and titanium as the refractory and further metals respectively.

Description

SPECIFICATION Improvements in or relating to metal coating This invention relates to a method for forming coherent wear-resistant coatings on the surface of substrates from refractory metals and refractory metal compounds.

There are a number of techniques for forming coatings on substrates including deposition of the coating material from the vapour phase. A recently developed vapour phase technique which has found favour in the deposition of coatings of refractory metals and alloys and electrically conducting compounds is known as sputter ion plating: the technique is usually operated with a simple DC glow discharge in an ionisable gas at a voltage dependent upon requirement, but typically of 1 KV, to transfer coating material by sputtering from source to substrate; sputtering readily generating the vapour of any of the refractory metals and their alloys. By contrast, in simple ion-plating, where vapour of the material to be deposited is generated by thermal evaporation from a crucible containing the material, problems are encountered in generating vapour of the refractory metals.

Sputter ion plating is a development of conventional sputtering wherein the source material is the principal cathode in an anode/cathode glow discharge system, the development being that of introducing the feature of substrate biassing characteristic of ion-plating to make the substrate a secondary cathode. In sputter ion plating with a negative potential of-i KV applied to the source material the negative bias applied to the substrate depends upon requirement but typically is -1001-125 V.Negative biassing of the substrate causes ion bombardment of the substrate surface, in addition to the relatively more powerful ion bombardment and sputtering of the principal cathode in the form of the source material: the substrate is surrounded by a space-charge region, termed the "Cathode fall", across which a substantial voltage can occur and ions which diffuse into the "cathode fall" are accelerated by the potential and bombard the substrate. Under conditions of simultaneous ionbombardment and vapour deposition, well adhered and dense metallic coatings have been shown to be formed.Aside from this technical advance, the advent of substrate biassing in sputter ion plating has been shown to result in enhancement of the glow discharge, an increase in the source current, and 3 corresponding increase in the deposition rate with no diminution in coating quality: the increase in source current may require that the substrate bias potential be above a threshold value.

Various workers investigating the deposition of refractory metal coatings by sputter ion plating have however reported that substrate bius has a pronounced effect on the morphology of the coatings formed. It has been recognised that without bias refractory metals and the refractory carbides nitrides and oxides of these metals, and other high melting metals and alloys such as chromium and stainless steel, tend to form dendritic-type coatings in which dendrites grow in coumnarfashion from substrate surfaces forming widespread interdcndritic porosity. However, when sufficient bias power is applied, it has been observed that smooth dense substantially porous-free coatings have been formed.Such dense consolidated coatings are essential in applications where surface toughness and wear resistance are of paramount importance. The bias power requirement for a predetermined coating growth rate varies depending upon the material being deposited and principally on the refractoriness of that material. It is believed that the mechanism by which negative substrate biassing causes a dense pore-free coating to form depends on a redistribution of the condensing material caused by resputtering and induced by the ion-bombardment. The term "ion-polishing" has bean coined to describe this physical mechanism which continuously prevents any excrescences from growing and causes the deposited material to form as a dense pore-free coating.

A!though Direct Current sputtering as generally used in sputter ion plating is a relatively low-rate coating process encouraging the slow formation of dense pore-free coatings without requiring excessive bias and the concomitant development of substrate heat in the deposition of a range of materials it has been found that power dissipution through overheating of substrates can seriously curtail allowable deposition rates. In the deposition of refractory materials requiring substantial bias potentials in attempts to produce dense pore-free coatings the problem of substrate heat and its build up can become a dominant fact'or particularly in view of the frequently prolonged treatment times: it will be apparent, for example, that the metallographic structure of a heat-treated steel substrate could be disturbed by prolonged exposure to elevated temperatures.In the sputter ion-plating of gun barrels, for instance, temperatures up to 6000C have been reported and it is believed that temperatures well in excess of this figure could be achieved in depositing the more refractory metals if the bias potential were to be sufficient to form dense pore-free coatings.

It has now been unexpectedly discovered that colluinnar growth in deposits during sputter ion plating of a refractory metal or a refractory metal compound can be reduced or substantially eliminated without the use of excessive substrate bias potentials, thereby incurring massive heating of the substrates, by co-daposilion of that metal or compound with one or more further metals which further one or more metals may be refractory metals.

Accordingly the invention resides in a method whereby the development of high substrate temperatures can be curtailed in the formation of a smooth dense substantially pore-free coating of a refractory metal or a refractory metal compound on a substrate by sputter ion plating wherein the source material consists of said refractory metal or metal compound and one or more further metals and the substrate bias potential is reduced compared to the bias potential required for plating said single refractory metal or metal compound alone. Said further metal may be another refractory metal, and a combination of refractory metals is to be preferred when surface coatings are required to have the properties of wear and erosion resistance associated with refractory metals their alloys and compounds.

In another aspect the invention provides a method of sputter ion-plating a smooth dense substantially porefree coating of a refractory metal or a refractory metal compound on the bore of a gun barrel wherein the source material consists of said refractory metal or metal compound and one or more further metals, which further metal may desirably be another refractory metal.

In this specification the term "refractory metai" means a metal having a melting point in excess of the melting point of iron (1 5350C) and includes the metals chromium, titanium, vanadium, rhodium, hafnium, ruthenium, niobium, iridium, molybdenum, osmium, tantalum, rhenium, tungsten, platinum, thorium, and zirconium; and the term "refractory metal compound" means a compound selected from the silicides, borides, nitrides, carbides and oxides of a "refractory metal" as defined above.

For deposition of a refractory metal onto the substrate the ionisable gas used should be one which does not react chemically with the substrate or the source material and should preferably be easily ionisable and have a high atomic number so as to form ions with a high momentum capable of ejecting metal atoms and ions from the source. A preferred gas is Argon, although other inert gases may be used. If a gas which is capable of reacting chemically with one or more of the meta! constituents of the source under the conditions of the glow discharge is mixed with the inert gas then compounds of the metal or metals may be formed in the discharge and deposited on the substrate: this process of compound formation at the substrate is known as "reactive sputtering".Columnar growth in the deposited coatings of these refractory metal compounds at nominal substratp bias potentials can be avoided if such a reactive gas mixture is used in conjunction with a source consisting of a refractory metal and a further metal.

Advantages may be gained from reactive sputtering in that a coating containing for example extremely hard or refractory compounds may be obtained. Titanium nitride in a matrix of tantalum may be prepared, for example, by the use of a source consisting of titanium and tantalum, the gas mixture being one of argon and nitrogen; (titanium nitride is formed in preference to tantalum nitride).

A gas pressure of ca. 1 00m Torr has been found to be suitable and this pressure may be maintained either statically or dynamically. A potential difference of between 200 and 2000 voits has been found to generate a suitable discharge in the ionisable gas in DC sputtering depending upon the geometry of the apparatus.

In one form of apparatus for carrying out the method of the invention, suitable for coating the inside surface of conducting tubes such as gun barrels, the source is in elongate form arranged along the axis of the tubular substrate. The source and tube may be contained within an enclosure containing a suitable gas or gas mixture, a primary discharge being generated between the source and an earth and a secondary discharge generated between the tube and the earth, the source and the substrate being negatively biased relative to the earth, or, alternativeiy, the enclosure may be dispensed with, the tube closed at both ends with airtight closures carrying appropriate electrical and gas inlet and exit connections, and a glow discharge established inside the tube itseif. In this case the secondary negative bias potential is applied to the tube.

The use of an axial source has been found to be particularly advantageous in that it enables a uniform discharge to be sustained in tubes with a length substantially greater than their diameter.

Typically if a potential of -1 000V is applied to the source, with a negative bias potential of - 125 V applied to the tubular substrate, discharges may be maintained in tubes up to 60 cm long. In the case of tubes with complex internal geometries, such as rifled gun barrels, and those with small substrate source distances, higher gas pressures may be needed than are normally used in sputter ion plating, but the exact pressure will depend upon the geometry of the apparatus and the metals involved etc.

It has been found advantageous to use a source consisting of a bundle of fine wires of the metals to be deposited. As the component metals may have different sputtering rates, the use of such a source ensures that the composition of the surface of the source exposed to the ionic bombardment remains constant as the metals are removed as vapour. Another effect of differential sputtering rates of the metals of the source is that the composition of the deposited coating may not always correspond to the composition of the source. Variation in the composition of the deposited coating is possible by altering the ratio of the metals in the source as appropriate.For many applications it has been found that the best coatings are formed by selecting deposition conditions which result in a coating which corresponds to a solid solution, for example one containing 10% by weight of tungsten in tantalum. The physical properties of any deposit formed during the coating process will of course have to be considered from the point of view of the application to which the coated substrate is to be put.

It is desirable to clean the surface of the substrate prior to the coating process by sputter cleaning.

This may be achieved by generating a glow discharge in the gas with the substrate arranged as the main cathode. In this way the surface of the substrate is bombarded with ions of the gas, which effectively remove surface impurities. If tenacious impurities such as metal oxides are present on the surface it is preferable to clean the surface by sputter cleaning in a reactive atmosphere, for example by sputter cleaning in an atmosphere of argon plus 5% of hydrogen. Other reactive gas mixtures may be used, depending on the type of impurities which are present on the surface.

The geometry of the source and the relative positioning of the source and substrate to achieve optimum coating will depend on the shape and size of the substrate. At the gas pressures usually used in the method of the invention, gas scattering in the vapour phase is normally sufficient to enable metal atoms etc, evaporated from the surface of the source to reach concavities and inturned areas of the substrate, and an unimpeded line of flight between the source and substrate is not normally necessary.

The practice of the invention has been found to produce tenacious, uniform, smooth coatings of refractory metals and refractory metal compounds on substrates of various shapes and sizes, at a convenient rate, without the generation of excessive substrate heat.

In the production of Ta: í 0%W wear-resistant coatings on the bores of gun barrels for example, smooth tenacious pore-free coatings have been formed with barrel temperatures in the range 200 to 6000C and satisfactory coatings can be obtained with temperatures as low as 2500C. The treatment may be applied to gun barrels of all sizes but is of especial significance to large barrels up to 21 ft in length and having wall thicknesses upto 4 ins.

It has generally been found that in the deposition of W coatings, for example, bias potentials cannot be raised sufficiently to create the degree of ion polishing needed to produce dense pore-free coatings free of columnar development: moreover such bias potentials have generally produced quite unacceptable levels of substrate heat.

It has already been stated that the practice of sputter ion plating results in an increase in source current, provided the substrate bias potential exceeds a threshold value, with a corresponding increase in the deposition rate compared with conventional sputtering. Measurement of the source current during the coating process is found to give the best indication of the deposition rate, aithough the source current and deposition rates are not directly proportional, and caution is needed in raisiny the source current as tco high a deposition rate may form powdery or loose coatings.

The invention will now be described by way of example only, with reference to the accompanying drawing and graphs in which, Fig 1 shows a schematic layout of an apparatus suitable for coating the inside of a metal tube using the method of the invention. Fig 2 shows the effect of various source potentials and secondary negative bias potentials on the source current using the apparatus of Fig 1 with a tube 1 6cm in length.

Fig 3 shows the effect of various source potentials and secondary negative bias potentials on the source current using the apparatus of Fig 1 with a tube 60cm in length.

With reference to Fig 1, a substrate comprises a hollow tube 1, the inner surface of which is to be coated with a tungsten/tantalum alloy. The tube 1 is mounted on an insulating support (not shown), inside a sheath 2. The tube 1 and sheath 2 are contained within an airtight metal walled enclosure 3 provided with an entrance port 4 and an exit port 5 connected to a vacuum pump (not shown). An electrical connector 6 enabling connection of the tube 1 to a DC electrical supply (not shown) passes through the sheath 2 and the wall of the enclosure 3 via the insulating plugs 7, that in the wall of the enclosure 3 making an airtight seal. Holes 8 are provided in the sheath 2 to allow access of gases introduced into the enclosure 3 via the entrance port 4 to the inside of the tube 1.A source 9 comprising a cylindrical bundle of tungsten and tantalum wires in a ratio of 9 tungsten wires to 1 tantalum wire passes axially along the tube 1 and through the wall of the enclosure 3 via the insulating airtight plug 1 0. An electrical connector 11 allows connection of the source 9 to a DC electrical supply (not shown). The cnclosure 3 is connected to an electrical earth 12.

In use the inner surface of the substrate 1 is first sputter cleaned. This may be carried out using either pure argon, or if tenacious surface impurities are present such as an oxide film by using a reactive gas mixture, for example 90% argon plus 10% hydrogen. To sputter clean the surface, the enclosure 3 is first evacuated to a high vacuum using the vacuum pump, and is then repressurised to a dynamic pressure of 0.2 mbar with the cleaning gas. A glow discharge is then established between the tube 1 and the source 9, the tube 1 being the cathode, a negative potential of ca. 1 000V being applied via the connector 6. The inner surface of the tube 1 is thereby bombarded with ions of the gas or gases present, and surface impurities are removed. If a reactive gas mixture is used, the reactive sputter cleaning is followed by a period of sputter cleaning using pure argon.

The electrical connection to the tube 1 is then broken, a negative potential of -1 000V is applied to the source 9 and a secondary negative bias potential of -1 25V is applied to the tube 1 via the connector 6, the enclosure 3 being pressurised with a dynamic pressure of ca. 0.2 mbar of pure argon. A glow discharge is thereby established inside the tube 1. Atoms, ions and atom-ion clusters of tungsten and tantalum are thereby ejected from the surface of the source, and are deposited on the inner surface of the tube 1, forming a tenacious and substantially non-columnar coating. When sufficient metal has been deposited on the tube 1, the discharge is stopped, air readmitted into the enclosure 3, and the coated tube 1 removed.

Although the method and apparatus above have been described with reference to the coating of the inner surface of tubes, it will be appreciated that by appropriate arrangements taking into account the shapes of source and substrate, substrates of other configurations may be used. Sources containing other ratios of tungsten and tantalum wires may also be used, for example 3:7, 5:5, and 7:3 Ta:W, to give tenacious non-columnar coatings of other compositions.

In all examples, in the practice of the invention, it has been found that substrate bias potentials of -1 00V/-1 25V have been quite effective in producing dense sound pore-free coatings. While substrate heat in the practice of the invention at these bias potentials develops to not insignificant levels it is generally not damaging to steel substrates, such as gun barrels, and effort is always made to keep the bias potential as low as possible consistent with achieving sound and dense non-columnar coatings. It is preferred to select metal combinations which are known to give the requisite wear-resistant coating but which also can form sound dense coatings with minimal values of bias potential.

With reference to Figs 2 and 3, the source current is plotted against the secondary negative bias potential over a range of source potentials. A pressure of 0.2 mbar of pure argon was used to establish the glow discharge. The information in both graphs was derived from use of the apparatus of Fig 1 using for Fig 2 a tubular substrate and axial source 1 6cm in length, and for Fig 3 60cm in length, in both cases 30mm inside diameter. The enhancement of the source current by the secondary negative bias potential can be clearly seen, particularly in the case of relatively low source potentials of 600 and 750 voits applied to the 1 6 and 60cm substrate respectively, when only extremely small source currents pass until the secondary negative bias potential is applied.

The effect of the secondary negative bias potential (Vb) upon the source current (Is), in mA, at various gas pressures (P and source potentials (Vs) is expressed as the gain in source current in Tables 1 and 2 which show the effect in tubular steel substrates 1 6cm and 60cm long respectively, each being 30mm inside diameter, the source comprising a cylindrical bundle cf tungsten and tantalum wires in a ratio of 9 tungsten wires to 1 tantalum wire.

Other methods of generating a glow discharge in the gas will be apparent to those skilled in the art and may be used in the practice of the invention, for example an RF discharge, or DC biased RF.

Methods involving RF are likeiy to be less satisfactory than a simple DC discharge, as RF methods normally require lower gas pressures and hence have a lower throwing power. In applications where there is a relatively large source -- substrate distance, or where the gas discharge needs to be established over a large volume for example in the case of a large substrate, less throwing power is required and RF methods may be suitable.

TABLE I 16cm tube, 30mm inside diameter

Vb (-V) # 0 75 100 125 150 175 200 P(mbar) Vs(V) Is(mA) Is gain Is gain Is gain Is gain Is gain Is gain 1000 0.2 0.2 I 0.2 I 0.2 I 0.2 I 0.2 I 0.2 I 1250 0.3 0.3 I 0.3 I 44 147 76 253 108 360 125 417 0.125 1500 0.35 0.35 I 54 154 79 26 109 311 140 400 1750 0.4 62 155 83 208 117 293 134 335 2000 0.5 82 164 117 234 131 262 150 300 100 0.3 0.3 1 0.3 1 0.3 1 57 190 75 252 110 367 1250 0.39 60 47.5 122 74 190 92 136 123 315 158 405 0.15 1500 5.4 56 10.4 82 15.2 112 20.7 140 25.9 1750 19 86 4.5 118 6.2 148 7.8 2000 28 118 4.2 145 15.9 500 0.2 0.2 10.2 1 0.2 1 0.2 1 12.5 62 25 125 600 0.2 0.2 1 0.2 1 0.2 1 18 90 38 190 55 275 750 0.2 0.2 1 8.2 41 22.5 112 36 180 62 310 88 440 0.175 1000 0.2 22 110 41 205 59 295 85.5 427 120 600 140 700 1250 7.3 52 7.1 69.2 9.5 99 13.6 130 17.8 162 22.2 1500 18.0 81.8 4.5 110 6.1 140 7.8 175 9.7 1750 30.0 114 3.8 142 4.7 2000 46.0 142 3.1 750 0.13 0.3 2 1.4 11 6.4 49 12 92 21.4 165 53 408 1000 6.6 23.6 4 40 6 63 9.5 92 14 120 18 142 21.5 0.2 1250 15 62 4.1 86 5.7 110 7.3 138 9.2 1500 28 94 3.4 120 4.3 149 5.3 1750 55 148 2.7 750 1.3 1.3 1 3.65 2.8 8.7 6.7 26.8 20.6 40.5 31.2 48.5 37.3 1000 8.5 29 3.4 49.6 5.7 69 8.1 94 11.1 126 14.8 150 7.6 0.25 1250 23 72 3.1 97.5 4.2 123 5.3 152 6.6 17.6 7.7 1500 44 112 2.5 600 2.6 1 2.9 1.1 3.0 1.5 5.6 2.2 9.0 3.5 18.0 6.9 0.3 750 6.3 9.7 1.5 14.2 2.25 22 3.5 50 7.9 93 14.8 150 23.8 1000 35 108 3.1 132 3.8 600 20 22.5 1.1 29.5 1.1 38.5 1.9 70 3.5 112 5.6 150 7.5 0.4 750 51 77 1.5 120 2.4 160 3.1 TABLE 2 60cm tube 30mm inside diameter

Vb (-V) # 0 75 100 125 150 175 200 P(mbar) Vs(V) Is(mA) Is gain Is gain Is gain Is gain Is gain Is gain 1250 0.35 0.35 1 0.35 1 0.35 1 0.35 1 150 428 170 486 1500 0.45 0.45 1 0.48 1.1 180 400 230 511 250 555 265 589 0.125 1750 0.55 1 80 145 240 436 285 518 2000 0.65 0.65 1 260 400 1000 0.35 0.35 1 0.35 1 0.35 1 0.35 1 98 280 135 386 1250 0.35 0.35 1 0.35 1 100 286 160 457 195 557 225 643 0.15 1500 0.55 63 114 180 327 245 445 280 509 1750 6.5 200 30.7 270 425 2000 78 275 3.5 750 0.25 1 0.25 1 0.25 1 0.25 1 0.25 1 70 280 88 352 1000 26 33 1.3 80 3.1 130 5 170 6.5 200 7.7 235 9.0 1250 63.5 150 2.4 200 3.1 240 3.8 270 4.3 0.175 1500 110 230 2.1 285 2.6 1750 150 295 2.0 2000 195 600 3.75 1.5 0.5 1.5 0.5 1.5 0.5 1.5 0.5 1.5 0.5 54 14.5 750 16 17.5 1.1 42 2.6 71 4.4 96 6 130 8.1 173 10.3 0.2 1000 65 140 2.2 180 2.8 200 3.1 230 3.5 260 4 300 4.6 1250 103 200 1.9 240 2.3 275 2.7 312 3.0 1500 155 275 1.8 500 3.2 1.2 0.4 1.2 0.4 1.2 0.4 1.2 0.4 5 1.6 26 8.1 600 12.0 6.0 0.5 6.5 0.6 19.0 1.6 42.5 3.5 74 6.2 120 10 0.25 750 40.5 45 1.1 68 1.7 105 2.6 138 3.4 182 4.4 230 5.7 1000 120 180 1.5 218 1.8 200 2.1 280 2.3 315 2.6 1250 182 280 1.5 312 1.7 500 9.2 10.5 1.1 10.5 1.1 10.5 1.1 14.6 1.6 38 4.1 60 6.5 600 37 0.8 42 1.1 66 1.1 93 2.5 155 4.2 215 5.8 0.3 750 85.5 110 1.3 133 1.6 170 2.0 225 2.6 278 3.3 1000 198 280 1.4 500 58 73 1.3 90 1.6 112 1.9 128 2.2 140 2.4 215 3.7 0.4 600 135 168 1.2 185 1.4 215 1.6 280 2.1 750 245 320 1.3

Claims (11)

1. A method whereby the development of high substrate temperatures can be curtailed in the formation of a smooth dense substantially pore-free coating of a refractory metal or a refractory metal compound on a substrate by sputter ion plating wherein the source material consists of said refractory metal or metal compound and one or more further metals and the substrate bias potential is reduced compared to the bias potential required for plating said single refractory metal or metal compound alone.

2. A method according to claim 1 wherein said further metal is a refractory metal.

3. A method according to either claim 1 or claim 2 wherein the source material is a bundle of wires consisting of wires of said refractory metal or metal compound and wires of said further metal.

4. A method according to any of claims 1 to 3 wherein, for predetermined glow discharge conditions of source and substrate potentials and ionisable gas pressure, said source material consists of said refractory metal or metal compound and said further metal in proportions to form a solid solution coating on said substrate.

5. A method according to any of claims 1 to 4 wherein the electrical potential is generated by a DC source.

6. A method according to any of claims 1 to 5 in which a steel substrate is plated.

7. A method according to any preceding claim in which the refractory metal is tungsten and the further metal is tantalum.

8. A method according to any preceding claim wherein the ionisable gas is an inert gas of high atomic number.

9. A method according to claim 8 wherein a gas capable of reacting chemically with said refractory metal or said further metal under the conditions of the glow discharge is mixed with said inert gas in forming a coating of a compound on the substrate.

10. A method according to claim 9 wherein the reactive gas is nitrogen and said refractory and further metals are tantalum and titanium.

11. A method of sputter ion plating a smooth dense substantially pore-free coating of a refractory metal or a refractory metal compound on the bore of a gun barrel wherein the source material consists of said refractory metal or metal compound and one or more further metals.